Photoelectric exposure meters

ABSTRACT

Photoelectric exposure meter including a selenium photocell coupled to an electrical measuring instrument calibrated in light values and a separate attachment including a second, cadmium sulfide type of photocell and a battery for extending the measuring range to lower light values, said attachment being mountable over the selenium cell on the exposure meter by a bayonet-type fixing including contact plates which engage other contacts on the exposure meter to couple the second cell to the measuring instrument whereby the latter is operated by current derived from the second photocell. A transistor amplifier is included in the attachment together with switching means in which the single-mercury cell battery forms the movable switch contact.

United States Patent [7 21 Inventor Gerliart Lothar Hamburger Welwyn,England [21] Appl. No. 835,682

[22] Filed June 23, 1969 [45] Patented Sept. 28, 1971 [73] AssigneeSangamo Weston Limited Enfield, Middlesex, England [32] Priority June25, 1968 [33] Great Britain [54] PHOTOELECTRIC EXPOSURE METERS 18Claims, 16 Drawing Figs.

[52] US. Cl 356/222, 356/224, 356/225, 356/226 [51] Int. Cl 60111142,G01 j 1/44 [50] Field of Search 356/222,

[56] References Cited UNITED STATES PATENTS 2,891,438 6/1959 Fuhrmann eta1 356/226 X 2,952,184 9/1960 Bakke et a1. 356/222 X Gosser, P.S.A.Journal, pp. 10-11, Dec., 1966 Primary Examiner-Ronald L. WibertAssistant Examiner-Warren A. Sklar Attorney-Young & Thompson ABSTRACT:Photoelectric exposure meter including a selenium photocell coupled toan electrical measuring instrument calibrated in light values and aseparate attachment including a second, cadmium sulfide type ofphotocell and a battery for extending the measuring range to lower lightvalues, said attachment being mountable over the selenium cell on theexposure meter by a bayonet-type fixing including contact plates whichengage other contacts on the exposure meter to couple the second cell tothe measuring instrument whereby the latter is operated by currentderived from the second photocell. A transistor amplifier is included inthe attachment together with switching means in which the single-mercurycell battery forms the movable switch contact.

PI-IOTOELECTRIC EXPOSURE METERS This invention relates to photoelectricexposure meters and is more particularly concerned with the provision ofan im proved arrangement having greater sensitivity and capable ofmeasuring an increased range of light intensity values or equivalentphotographic stop, exposure or other values.

In accordance with one aspect of this invention there is provided for aphotoelectric exposure meter comprising a first casing having a firstlight entry aperture in a wall thereof, attachment holding means in thevicinity of the said first light entry aperture, a first photosensitivedevice located within said casing for exposure to light entering throughsaid first light entry aperture, an electrical measuring instrumentcalibrated in light or equivalent photographic values, electric circuitmeans connecting said measuring instrument with said firstphotosensitive device to be energized by a current which varies inamplitude in accordance with the degree of illumination of said firstphotosensitive device by light entering said first light entry apertureand external electric connector means on said casing and connected tosaid electrical measuring instrument, an attachment for extending themeasurement range of said meter to light values of lower intensity,which attachment comprises a second casing having a second light entryaperture in a 'wall thereof, a second photosensitive device locatedwithin said second casing for exposure to light entering through suchsecond light entry aperture, external electrical connector meansconnected to said second photosensitive device and attachment means forcooperating with said attachment holding means on said first casing tohold said attachment in a position which closes said first light entryaperture against the passage of light therethrough onto such firstphotosensitive device and with said external electric connector meanscooperating with the external electrical connector means of said firstcasing to connect said second photosensitive device to said electricalmeasuring instrument in an electric circuit by means of which saidmeasuring instrument is energized in accordance only with the degree ofillumination of said second photosensitive device.

In accordance with another aspect of the invention an arrangement formeasuring a plurality of different ranges of light intensity valueseither in direct terms of light intensity or as equivalent photographicexposure or other values comprises a photoelectric exposure meterincluding a first casing having a first light entry aperture in a wallthereof, a first photosensitive device located within said casing forexposure to light entering through said light entry aperture, anelectrical measuring instrument calibrated in light or equivalentphotographic values, electric circuit means connecting said measuringinstrument with such first photosensitive device to cause energizationof such measuring instrument by a current which varies in amplitude inaccordance with the degree of illumination of said first photosensitivedevice by light entering said light entry aperture, attachment holdingmeans on said first casing in the vicinity of said first light entryaperture and external electric connector means electrically connected tosaid electrical measuring instrument and an attachment arranged fordetachable mounting on said first casing and comprising a secondphotosensitive device, attachment means cooperating with said attachmentholding means of said first casing to secure said attachment to saidfirst casing in a position completely obscuring said first light entryaperture from the entry of light therethrough onto said firstphotosensitive device and external electric connector means cooperatingwith said external electric connector means connecting said secondphotosensitive device into the circuit of said electrical measuringinstrument whereby said measuring instrument is energized solely inaccordance with the illumination incident upon said secondphotosensitive device when said attachment is in position and wherebysaid measuring instrument is energized only in accordance with thedegree of illumination incident upon said first photosensitive devicewhen said attachment is removed from said meter.

In order that the nature of the invention may be more readilyunderstood, certain embodiment of the invention will now be described byway of illustrative example only and with reference to the accompanyingdrawings in which:

FIG. 1 is a circuit diagram showing one known form of photoelectricexposure meter and an attachment therefor constructed in accordance withthe invention.

FIG. 2 is a circuit diagram of an alternative form of attachment also inaccordance with the invention.

FIG. 3 is a circuit diagram of a preferred form according to theinvention of an arrangement comprising a photoelectric exposure metersuitable for conventional use and an attachment therefor according tothe invention.

FIG. 4 is a front perspective view, FIG. 5 is a side elevation, FIG. 6is an underside elevation and FIG. 7 is a rear elevational view of oneparticular form of attachment embodying the invention.

FIG. 8 is a diametral cross-sectional view through the attachment ofFIGS. 4-7

FIGS. 9 and 10 are fragmentary detail views of parts of the structureshown in FIG. 8.

FIG. 11 is a front perspective view, FIG. 12 is a rear elevational view,and FIG. 13 is a side elevational view of one form of photoelectricexposure meter particularly adapted for use with an attachment as shownin FIGS. 4-10, the said attachment being indicated in chain-dotted linesin its operative position in FIGS. 12 and 13.

FIG. 14 is a fragmentary perspective view illustrating theinterengagement relationship between the exposure meter of FIGS. 11-13and the attachment of FIGS. 4-10 while FIGS. 15 and 16 are fragmentaryperspective views showing in detail the construction of theinterengagement and circuit connection means on the exposure meter andon the attachment.

Referring first to FIG. 1, the dotted rectangle 10 indicates oneconventional form of photoelectric exposure meter which comprises aphotosensitive device 11 in the form of a current generating photocell,e.g. a selenium cell, which is connected to supply its output current toan electrical measuring instrument l2, e.g. of the moving coil type,which is appropriately calibrated in suitable photographic terms such aslight values. The photocell 11 is arranged to be exposed to lightincident in the direction of the arrows 13. Usually a baffle orequivalent ray direction selective means is provided to limit the angleof incidence of the light falling upon the cell while such baffle ordirection selective means often incorporate adjustable lightrestrictingmeans to allow provision of one or more light measurement ranges by themeter 12. Such conventional exposure meters generally have a measurementrange which terminates, at the lower intensity end, at a brightnessvalue of around 0.2 candles/sq. ft.

In order to allow extension of the measurement range of the meter 10 tostill lower brightness values the present invention provides anattachment indicated by the dotted line rectangle 14 and comprising asecond photoelectric device in the form of a second photocell 15. Thiscell is conveniently of the photoconductive type, e.g. a cadmium sulfidecell, and is connected in series with a current source in the form of adry battery 16 of appropriate voltage, the series arrangement of cell 15and battery 16 being connected across a pair of externally directedcontacts 17 and 18. These contacts 17 and 18 are mounted upon the casingof the attachment 14 and are adapted to enter into engagement withcooperating contacts 19 and 20 mounted upon the exposure meter 10. Thecontacts 19, 20 provide connections to the measuring instrument 12 ofthe meter 10 whereby such instrument completes the circuit through thephotocell 15 and the battery 16 when the attachment 14 is placed inposition on the meter 10.

The act of applying the attachment 14 to the exposure meter 10 so as toengage the contacts 17, 18 with the contacts 19, 20 is arrangedautomatically to mask the photocell 11 of the meter 10 from all light.In consequence of the photocell 11 being completely darkened it does notprovide any output current to the measuring instrument 12 and is of veryhigh resistance whereby its shunting effect on such measuring instrument12 is negligible. Any light incident on the cell 15, as indicated by thearrows 21, now causes alteration of the resistance of the photocell 15thereby to alter the current flow from the battery 16 through themeasuring instrument 12. Such an arrangement can readily provide anextension of the sensitivity of the exposure meter to the region of0.002 candles/sq. ft.

FIG. 2 shows a modified form of the attachment 14 in which a currentamplifying transistor 22 is employed. The photocell is now connected inseries between the positive terminal of the battery 16, which terminalis also connected to contact 17 and the base of the transistor 22. Thelatter has its collector connected to the contact 18 and its emitterconnected to the negative terminal of the battery 16. The photocell 15now serves to control the base current flow to the transistor and thecollector current of such transistor serves to operate the measuringinstrument 12 of the associated exposure meter 10 which is assumed to beconstructed in a manner identical with that shown in FIG. 1. The use ofan amplifying transistor permits the voltage of the current sourcebattery 16 to be greatly reduced for any given full scale deflectionvalue of the instrument 12; in many instances such battery 16 need beonly a single-mercury cell of about 1.35 volts.

FIG. 3 shows a further and preferred modification according to theinvention which is particularly adapted to the commercial constructionof attachments 14 as accessories which can be sold separately and at adifferent time and place from the exposure meters 10 with which they areto be used. Such production requirement means that each attachment maybe applied to any specimen of the particular type of exposure meter 10for which it is designed while maintaining any appropriate degree ofaccuracy.

In this preferred embodiment the attachment 14 has the second photocell15, the battery 16 and the amplifying transistor 22 arranged in a mannersimilar to that of FIG. 2. A three-position switch 23 is provided toallow completion of a circuit including the instrument 12 either throughthe photocell 15 or through a fixed resistance Rc in addition to anintermediate position which provides an off or out-of-use condition. Thevalue of the resistance Re is chosen to provide deflection of themeasuring instrument 12 of the exposure meter 10 to a chosen referencepoint on its scale when the voltage of the battery 16 is at a suitablelevel and thereby serves to afford means for checking the voltage ofsuch battery 16. A fixed resistance Rs is connected in series with thebase connection of the amplifying transistor 22 and serves to increasethe total light range which can be embraced by the deflection range ofthe instrument 12 of the associated exposure meter 10. The calibrationof the scale of the meter 12 is frequently in terms of spot values andthe number of such spot markings can be increased and the linearity oftheir mutual spacing improved by appropriate choice of the value of theresistance Rs. A further fixed value resistance Rf is provided in serieswith the connection between the emitter of the transistor 22 and thenegative terminal of the battery 16 to provide a measure of currentfeedback which is operative to reduce the effects of differences betweenthe characteristics of the respective transistors 22 which are used inproduction of a batch of the attachments. Such feedback resistance alsoimproves the linearity of the calibration markings towards the upper endof the scale range of the instrument 12. A further fixed valueresistance Rsh which is, effectively connected across the instrument 12when the attachment is in use, serves to shunt away excess current whichmay be caused by the transistor gain being higher than a certainpredetermined minimum.

In the associated exposure meters 10 with any one of which any oneattachment 14 may be used, production variations of the characteristicof the photocells 11 and variations in the sensitivities of theassociated instrument 12 are met by incorporating a high value shuntresistance R1 for connection across the meter 12 whenever the attachment14 is in use while a series resistance R2 is provided between the fixedcontact 19 of the meter and the common connection point between thepositive terminal of the meter and the related terminal of the photocell1]. Separate contacts 20:: and 20b, normally insulated from one another.are provided on the exposure meter 10 and arranged each to be engaged bya common contact 18 on the attachment 14. By these means the contacts20a and 20b become bridged only when the attachment 14 is in place, sothat both of the resistances R1 and R2 are inoperative whenever theexposure meter 10 is being used without the attachment. The applicationof an attachment to a photo exposure meter with the consequentengagement of contacts 17 and 18 with contacts 19 and 20 0, 20bautomatically brings the resistance R] into shunting connection acrossthe instrument 12 and places the resistance R2 in series with thebattery and the second photocell 15 through the transistor 22. Theresistances R1 and R2 have values chosen to make each exposure meterpresent predetermined resistance or deflection sensitivity value to anyattachment 14.

Referring now to FIGS. 4-10, the particular form of the at tachment 14shown therein comprises a shallow cylindrical casing 30 the rearwardwall 31 of which is provided with a nearly circular axially directedextension 32 carrying two radially directed contact plates 17 and 18which serve the dual purpose of locking the attachment in position uponthe associated exposure meter and simultaneously establishing thenecessary electrical connections between the attachment and the meterand also within the meter itself as already described above withreference to FIG. 3. The front wall 33 of the attachment, which is theone caused to face the field whose illumination is to be measured,includes an exposure calculator mechanism 24 surrounding a centralaperture 35 (see F IG. 8). As shown more clearly in FIG. 8, the latteris at one end of a central hollow stem 36 having an internal bore ofstepped profile presents a series of progressively smaller diameter andnonreflecting zones leading to an inner aperture 37 lying immediatelyadjacent the light sensitive region of a cadmium sulfide photocell 15.The circuit components as shown within the dotted line rectangle 14inFIG. 3, except the photocell 15 and switch 23, are mainly supported uponan annular-shaped printed circuit board 38 which is disposed around thecentral hollow stem 36 and secured in position as shown. The exposurecalculator 24, in the form of superimposed rotatable metal plates 39, 40is also mounted around the central stem 36.

The plate 40 may be locked in any one of a number of adjustmentpositions by means of a plunger 41 entering one of a number of spacedholes in the plate. Such plunger 41 can be withdrawn against the actionof a spring 42 by a movable thumb piece 43.

The switch shown at 23 in FIG. 3 is constituted in the construction ofFIGS. 4-10, by a mechanism wherein the battery 16 in the form of aminiature mercury cell of button-shape serves also as the movable orrotor contact. This cell 16 is located within a retaining pocket in amember 44 of moulded plastic insulating material which can be movedalong an arcuate path by means of an outwardly projecting serrated orroughened finger piece 45 projecting through an arcuate slot 46 in theouter wall of the casing 30 to permit its operation by an arcuatemovement about the axial line of the casing in either direction from acentral off position which is that shown in FIG. 4. The mercury cell 16is located between opposed spring contact arms 47, 48 (FIG. 9) whichmake connection respectively with its opposite end terminals, whilefurther resilient switch contact arms as shown at 49, S0 in FIG. 9,serve also to contact the cell when in either one of its two oppositeend positions to establish connections in accordance with the circuitdiagram of FIG. 3. A spring detent as shown at 51 in FIG. 10 cooperateswith notches or apertures in the casing 30 to lock the switch in anychosen position.

A central snap-on cap 25 is provided for closing the outer end apertureof the stem leading to the cell 11 so as to prevent ingress of dirt andother foreign material when not in use,

while a further outer bell-shaped cover 52, also of plastic material,may be provided to cover the whole of the attachment by engagement of acentral recess 53 therein with the outer edge of the cover 25.

Referring now to FIGS. 11, 12 and 13, the cooperative photoelectricexposure meter consists of a main outer casing 56 having a front wall 57containing a window aperture 58 for viewing the pointer of theincorporated electrical measuring instrument. This front wall 57 alsocarries the normal exposure calculator 59 dealing with the range oflight values which can be measured by the meter in its original,unmodified form.

The opposite or rear wall 60 of the casing 56 is provided with asubstantially circular opening 61 surrounding the light sensitive areaof the internal selenium cell which forms the inbuilt photosensitivedevice of the exposure meter. A masking flap 62 arranged to fit withinsuch opening 61 and hinged at 63, contains a number of spaced apertures64 therein and operates as a removable mask for reducing the sensitivityof the exposure meter and thereby extending the range of light valueswhich can be measured thereby in an upward or increasing light valuedirection.

As shown clearly in FIGS. 14, and 16, the attachment 14 is arranged byits extension 32 to interfit with the aforesaid circular opening 61around the window of the cell in the exposure meter by an arrangement inwhich the projecting radial contact plates 17, 18 enter into slots 65and 66 formed in the body of the meter casing. Such radial platesoperate, after slight rotary movement oftheattachment to secure itbayonet fixing fashion to make electrical connection with the springblade contacts 19 and a, 20b thereby to establish the circuitarrangement shown in FIG. 3. As will be appreciated the insertion ofsuch attachment into the recess 61 of-the normal exposure meterautomatically prevents access of light to the selenium cell embodied inthe exposure meter. The attachment 14 is held located in its mountedposition by the-entry of resilient detents 67 into recesses 68 formed inthe rear wall of the exposure meter casing.

While the invention has been more particularly described with referenceto one specific form of exposure meter and using one particular designof attachment device therefor, it will be understood that numerousmodifications may be made and an attachment device of suitable formarranged for cooperating withsubstantially any type of exposure meter.

,1 claim:

1. For a photoelectric exposure meter comprising a first casing having afirst light entry aperture in a wall thereof, attachment holding meansin the vicinity of the said first light entry aperture, a firstphotosensitive device located within said casing for exposure to light.entering through said first light entry aperture, an electricalmeasuring instrument calibrated in light or equivalent photographicvalues, electric circuit means connecting said measuring instrument withsaid first photosensitive device to be energized by a current whichvaries in amplitude in accordance with the degree of illumination ofsaid first photosensitive device by light entering said first lightentry aperture and external electric connector means on said casing andconnecting to said electrical measuring instrument, the improvement.comprising an attabhment for extending the measurement range of saidmeter to light values of lower intensity, which attachment comprises asecond casing having a second light entry aperture in a wall thereof, asecond photosensitive device located within said second casing forexposure to light entering through such second light entry aperture,external electrical connector means connected to said secondphotosensitivedevice and attachment means for cooperating with saidattachment holding means on said first casing to hold said attachment ina position which closes said first light entry aperture against thepassage of light therethrough onto such first photosensitive device andwith said external electric connector'means cooperating with theexternal electrical connector means of said firstcasingto connect saidsecond photosensitive device to said electrical measuring instrument inan electric circuit by means of which said measuring instrument isenergized in accordance only with the degree of illumination of saidsecond photosensitive device.

3. An arrangement for measuring a plurality of different ranges of lightintensity values which comprises a photoelectric exposure meterincluding a first .casing having a first light entry aperture in a wallthereof, a first photosensitive device located within said casing forexposure to light entering through said light entry aperture, anelectrical measuring instrument calibrated in light or equivalentphotographic values, electric circuit means connecting said measuringinstrument with such first photosensitive device to cause energizationof such measuring instrument by a current which varies in amplitude inaccordance with the degree of illumination of said first photosensitivedevice by light entering said light entry aperture, attachment holdingmeans on said first casingin the vicinity of said first light entryaperture and external electric connector means electrically connected tosaid electrical measuring instrument and an attachment arranged fordetachable mounting on said first casing and comprising a secondphotosensitive device, attachment means cooperating .with saidattachment holding means of said first casing to secure said attachmentto said first casing in a position completely obscuring said first lightentry aperture from the entry of light therethrough onto said firstphotosensitive device and external electric connector means cooperatingwith said external electric connector means connecting said secondphotosensitive device into the circuit of said electrical measuringinstrument whereby said measuring instrument is energized solely in.accordance with the illumination incident upon said secondphotosensitive device when said attachment is in position and wherebysaid measuring instrument is energized onlyin accordance with the degreeof illumination incident upon said first photosensitive device when saidattachment is removed from said meter.

3. An attachment according to claim 1 in which said secondphotosensitive device is of the photoresistive type and in which saidattachment also carries a current source.

4. An attachment in accordance with claim 3 in which said secondphotosensitive device is a cadmium sulfide cell.

5. An attachment in accordance with claim 3 in which said attachmentincludes manually operable switch means by which said current source maybe connected into operative circuit relationship with said secondphotosensitive device, disconnected or connected into a voltage testcircuit which includes said electrical measuring instrument.

6. An attachment according to claim 1 which includes a transistorcurrent amplifier for amplifying the output from said secondphotosensitive device.

7. An attachment according to claim 1 which carries an exposurecalculator mechanism for dealing withthe additional illuminationmeasurement range provided thereby.

8. An attachment according to claim 1 in which said .attachment holdingmeans on said first casing and said .attachment means on said secondcasing comprise a bayonettype coupling.

9. An attachment according to claim 8 in which said bayonet-typecoupling includes projecting metal parts which servealso as saidexternal electric connectormeans on said first and second casings.

10. An arrangement according to claim 2 in .which said secondphotosensitive device is of the photoresistive type and in which saidattachment also includes a current source.

11. An arrangement according to claim 10 in which said secondphotosensitive device is a cadmium sulfide cell.

12. An arrangement according to claim 10 in which said attachmentincludes manually operable switch means'for connecting and disconnectingsaid current source into and from operative circuit relationship withsaid second photosensitive device and for connecting such current sourceintoa voltage test circuit which includes said electricalmeasuringinstrument.

13. An arrangement according to claim 2 in which said attachmentcomprises a transistor current amplifier operative to amplify the outputfrom said second photosensitive device.

14. An arrangement according to claim 2 in which said attachmentcomprises an exposure calculator mechanism for dealing-with theadditional illumination measurement range provided thereby.

15. An arrangement according to claim 2 in which said attachment holdingmeans on said first casing and said attachment means on said secondcasing comprise a bayonettype coupling.

16. An arrangement according to claim in which said bayonet-typecoupling includes cooperating metal parts which

1. For a photoelectric exposure meter comprising a first casing having afirst light entry aperture in a wall thereof, attachment holding meansin the vicinity of the said first light entry aperture, a firstphotosensitive device located within said casing for exposure to lightentering through said first light entry aperture, an electricalmeasuring instrument calibrated in light or equivalent photographicvalues, electric circuit means connecting said measuring instrument withsaid first photosensitive device to be energized by a current whichvaries in amplitude in accordance with the degree of illumination ofsaid first photosensitive device by light entering said first lightentry aperture and external electric connector means on said casing andconnecting to said electrical measuring instrument, the improvementcomprising an attachment for extending the measurement range of saidmeter to light values of lower intensity, which attachment comprises asecond casing having a second light entry aperture in a wall thereof, asecond photosensitive device located within said second casing forexposure to light entering through such second light entry aperture,external electrical connector means connected to said secondphotosensitive device and attachment means for cooperating with saidattachment holding means on said first casing to hold said attachment ina position which closes said first light entry aperture against thepassage of light therethrough onto such first photosensitive device andwith said external electric connector means cooperating with theexternal electrical connector means of said first casing to connect saidsecond photosensitive device to said electrical measuring instrument inan electric circuit by means of which said measuring instrument isenergized in accordance only with the degree of illumination of saidsecond photosensitive device.
 2. An arrangement for measuring aplurality of different ranges of light intensity values which comprisesa photoelectric exposure meter including a first casing having a firstlight entry aperture in a wall thereof, a first photosensitive devicelocated within said casing for exposure to light entering through saidlight entry aperture, an electrical measuring instrument calibrated inlight or equivalent photographic values, electric circuit meansconnecting said measuring instrument with such first photosensitivedevice to cause energization of such measuring instrument by a currentwhich varies in amplitude in accordance with the degree of illuminationof said first photosensitive device by light entering said light entryaperture, attachment holding means on said first casing in the vicinityof said first light entry aperture and external electric connector meanselectrically connected to said electrical measuring instrument and anattachment arranged for deTachable mounting on said first casing andcomprising a second photosensitive device, attachment means cooperatingwith said attachment holding means of said first casing to secure saidattachment to said first casing in a position completely obscuring saidfirst light entry aperture from the entry of light therethrough ontosaid first photosensitive device and external electric connector meanscooperating with said external electric connector means connecting saidsecond photosensitive device into the circuit of said electricalmeasuring instrument whereby said measuring instrument is energizedsolely in accordance with the illumination incident upon said secondphotosensitive device when said attachment is in position and wherebysaid measuring instrument is energized only in accordance with thedegree of illumination incident upon said first photosensitive devicewhen said attachment is removed from said meter.
 3. An attachmentaccording to claim 1 in which said second photosensitive device is ofthe photoresistive type and in which said attachment also carries acurrent source.
 4. An attachment in accordance with claim 3 in whichsaid second photosensitive device is a cadmium sulfide cell.
 5. Anattachment in accordance with claim 3 in which said attachment includesmanually operable switch means by which said current source may beconnected into operative circuit relationship with said secondphotosensitive device, disconnected or connected into a voltage testcircuit which includes said electrical measuring instrument.
 6. Anattachment according to claim 1 which includes a transistor currentamplifier for amplifying the output from said second photosensitivedevice.
 7. An attachment according to claim 1 which carries an exposurecalculator mechanism for dealing with the additional illuminationmeasurement range provided thereby.
 8. An attachment according to claim1 in which said attachment holding means on said first casing and saidattachment means on said second casing comprise a bayonet-type coupling.9. An attachment according to claim 8 in which said bayonet-typecoupling includes projecting metal parts which serve also as saidexternal electric connector means on said first and second casings. 10.An arrangement according to claim 2 in which said second photosensitivedevice is of the photoresistive type and in which said attachment alsoincludes a current source.
 11. An arrangement according to claim 10 inwhich said second photosensitive device is a cadmium sulfide cell. 12.An arrangement according to claim 10 in which said attachment includesmanually operable switch means for connecting and disconnecting saidcurrent source into and from operative circuit relationship with saidsecond photosensitive device and for connecting such current source intoa voltage test circuit which includes said electrical measuringinstrument.
 13. An arrangement according to claim 2 in which saidattachment comprises a transistor current amplifier operative to amplifythe output from said second photosensitive device.
 14. An arrangementaccording to claim 2 in which said attachment comprises an exposurecalculator mechanism for dealing with the additional illuminationmeasurement range provided thereby.
 15. An arrangement according toclaim 2 in which said attachment holding means on said first casing andsaid attachment means on said second casing comprise a bayonet-typecoupling.
 16. An arrangement according to claim 15 in which saidbayonet-type coupling includes cooperating metal parts which serve alsoas said external electric connector means on said first and secondcasings.
 17. An arrangement according to claim 16 in which the operativecircuit within said exposure meter is altered when said attachment isapplied thereto and in which the application of said attachment isarranged automatically to effect the necessary circuit alteration. 18.An arrangement according to claim 17 in which the required circuitalteratiOn is effected by bridging at least two contacts of saidexternal electric connector means of said exposure meter with one ofsaid bayonet-type connector members.